Problem: Simplify the following expression: $n = \dfrac{1}{10r} - \dfrac{1}{9r}$
Answer: In order to subtract expressions, they must have a common denominator. The smallest common denominator is the least common multiple of $10r$ and $9r$ $\lcm(10r, 9r) = 90r$ $ n = \dfrac{9}{9} \cdot \dfrac{1}{10r} - \dfrac{10}{10} \cdot \dfrac{1}{9r} $ $n = \dfrac{9}{90r} - \dfrac{10}{90r}$ $n = \dfrac{9 -10}{90r}$ $n = \dfrac{-1}{90r}$